1. Field of the Invention
The present invention relates to a stainless cast alloy steel for use at low temperatures.
More particularly, the present invention provides a stainless cast alloy steel for use at low temperatures as a casting material for use in fabricating apparatus for use in a LNG (Liquid Natural Gas) plant or the like to be used under a low temperature atmosphere (from room temperature to 0.degree. K.), for example, valves, pumps, compressor bodies, bonnets, casings or the like, particularly portions that deform by martensitic transformation which must be avoided during production or use, e.g., ground joint portions, airtight portions, sliding portions or the like.
2. Description of the Prior Art
Heretofor, ASTM CF 8 (18 Cr-8Ni) has been mainly used as the stainless cast alloy steel for low temperature environments. However, this material has the defect that, when exposed to low temperature atmosphere, a volume change due to martensitic transformation occurs in a short period of time, whereby the material deforms, and, as a result, deformation sometimes leads to cracking and leakage of explosive vapors. In order to prevent such deformation, various attempts have been made, e.g.: (1) the upper value of the nickel content of the standardized range of CF 8 is used instead of the lower value (actually, more than 9.5-10% by weight); (2) sub-zero treatment (immersion in liquid nitrogen at - 196.degree. C. for 0.5 to 1.5 hrs) is conducted several times; (3) expensive CF 8M (18 Cr-12 Ni-2Mo) is used and so on. However, these solutions do not provide a satisfactory result, rather, they increase the cost of products formed therefrom.
Further, as a two-phase alloy containing ferrite, several stainless alloy steels have been developed for the purpose of improving resistance to stress, corrosion cracking or the like as disclosed in Japanese Pat. No. 602,636 and French Pat. No. 1,422,764, but these are stainless alloy steels as opposed to stainless cast alloy steels of the present invention, and generally crystallize a ferrite phase in an amount of at least about 40% by volume (in the structure of the steel).